It was only yesterday morning, after I’d written the first draft of this piece, that the Pfizer vaccine was provisionally approved by Australia Therapeutic Goods Administration (I suspect the TGA was shouted at by a senior politician).

The Morrison government has ordered 54 million AstraZeneca doses to be manufactured in Australia, enough to cover every citizen with the required two shots.

But it has ordered only enough Pfizer - which will arrive first - to cover five million people. There is a priority list for receiving this vaccine: front line healthcare and emergency workers and folks in aged care being at the top.

(I assume politicians, being loyal Australians, will receive the locally produced AstraZeneca and, being committed to the people, will place themselves well down the priority list. Of course, I joke.)

I intend to get the jab as soon as I can and I would prefer it to be the highly efficacious Pfizer rather than the poorer performing AstraZeneca, the first guaranteeing me almost 100% protection; the second more of a longshot.

But will I qualify to receive the more efficacious and early arriving Pfizer or have to wait for the lesser breed? I did some figuring (the numbers are rounded for clarity).

There are almost 26 million Australians and four million of us are aged over 65. The 1A priority group designated by the government covers quarantine personnel and frontline health workers (one million at the most) and people in aged care (about 300,000). Let's say that's 1.3 million of the five million available Pfizer two-shot jabs accounted for (each person needs two doses to maximise protection).

The next group of 1B priorities include emergency service workers (150,000), people aged over 70 (2.7 million, one of whom is me) and Indigenous Australians over 55 and some medically-compromised younger folks (100,000 tops). Let's say another three million people.

So now I've accounted for 4.3 million doses of the superior Pfizer vaccine. So who gets the remaining 700,000?

We move to priority group 2A. Top here are people aged 60-69. They number three million. And that poses a problem.

They can’t all get the more efficacious Pfizer, so 2.3 million will have to settle for the less efficacious AstraZeneca. A divisive proposition, one would think.

Well, having led you through that maze, my guess is that, apart from the high need group in 1A, the Pfizer vaccine will not be distributed according to the priority list.

In fact, I detect that the Australian government's rhetoric is already conflating the two vaccines suggesting they are both of similar efficacy. This has got many scientists and researchers worried.

This is because, assuming the AstraZeneca shot is effective in just 62% of cases, it will not build up the 'herd immunity' required to render Covid no more threatening than a bad case of influenza.

It is considered that herd immunity for Covid will kick in when at least 70% - and more likely 80% - of a population is immune.

In short, it seems Pfizer can achieve herd immunity and AstraZeneca cannot. So those five million Pfizer doses on order are not enough to do the trick and AstraZeneca (unless it improves it efficacy) can't either.

Of course both vaccines are safe for the vast majority of people and any protection against Covid is better than none. But we need to view the rosy predictions floated by the Australian government with suspicion.

And perhaps we can hope that one day prime minister Morrison, health minister Hunt and their senior health officer disciples will level with us. Right now we're living in a cloud of often conflicting and always less than complete information.

(Footnote: Nothing in this piece is health advice. My only association with the medical profession is that I am firmly in its grip.)